Studies on dimerisation of tocopherols under the influence of methyl linoleate peroxides

Abstract

The stability of plant oils is related to the level of polyunsaturated fatty acids and the presence of native antioxidants--especially tocopherols. During storage, lipids or the fat products undergo oxidation and tocopherol dimers and trimers are formed. These compounds possess reducing and antioxidant properties and participate in oxidation clearly inhibiting this process. In the present study, the correlation between levels of peroxides formed during autoxidation of methyl linoleate and simultaneous decomposition of tocopherols was examined. The peroxide value was investigated. Quantities of decomposed tocopherols and formation of their dimers were determined by high-performance liquid chromatography (HPLC). Mass spectrum analysis confirmed thatthe analysed compounds were dimes. Dimerisation of gamma-T begins at the smaller quantity of the methyl linoleate peroxides than dimerisation of delta-T. At the beginning of methyl linoleate autoxidation dimerisation of gamma-T in relation to its loss was smaller. The quantity of gamma-T dimers with ether bonds in total dimers pointed to faster binding of phenoxy radicals than transformation into the phenyl ones. delta-T dimers with phenyl bonds constitute about 65% of the total. The quantity of peroxides in methyl linoleate, necessary for quantitative and qualitative changes of homologous tocopherols, decreased from delta-T to alpha-T.

Identification of α-Tocotrienolquinone Epoxides and Development of an Efficient Molecular Distillation Procedure for Quantitation of α-Tocotrienol Oxidation Products in Food Matrices by High-Performance Liquid Chromatography with Diode Array and Fluorescence Detection

“…The lower enrichment factors of the analytes in lard were due to the higher evaporator temperature, which led to a higher proportion of interfering volatile components in the distillate. The recovery rates of the tocochromanols found in our experiment are comparable to those determined in other studies involving molecular distillation or solvent extraction of tocochromanols. ,,, However, no information on the enrichment factors or the recovery rates of the tocopherol oxidation products is given for the conventional extraction procedures for tocopherol oxidation products such as solvent extraction, solid phase extraction, or saponification. ,,,, Some authors assumed recovery rates similar to those determined for T. , Applying trypsin incubation and solvent extraction, Faustman et al . determined recovery rates of internal deuterated standards of α-T, α-TQ, α-TQ-4a,5-E, and α-TQ-7,8-E to be 95–111%.…”

“…16 However, data from the literature are contradictory for the formation of tocochromanol dimers and trimers in oxidized lipid matrices. Some previous studies do indicate the formation of tocochromanol dimers and trimers in lipid matrices, 21,24,44 but these results are not in agreement with those of Yamauchi and coauthors, 45 who showed that α-T suppresses the formation of methyl linoleate hydroperoxides in methyl linoleate and that the main oxidation products of α-T are epoxy-8a-hydroperoxy-α-tocopherones and 8a-alkylperoxyα-tocopherones, which convert to α-TQE and α-TQ. The α-T dimers and trimers are formed only in a reaction mixture containing large amounts of α-T.…”

“…18,20,27,29 However, no information on the enrichment factors or the recovery rates of the tocopherol oxidation products is given for the conventional extraction procedures for tocopherol oxidation products such as solvent extraction, solid phase extraction, or saponification. 17,21,23,24,26 Some authors assumed recovery rates similar to those determined for T. 18,20 Applying trypsin incubation and solvent extraction, Faustman et al 22 determined recovery rates of internal deuterated standards of α-T, α-TQ, α-TQ-4a,5-E, and α-TQ-7,8-E to be 95−111%. Furthermore, no data from the literature are available for the extraction efficiency of tocochromanol oxidation products by molecular distillation.…”

“…Measurement of tocochromanol oxidation products in food requires extraction and concentration prior to analysis by HPLC. The extraction or enrichment of α-T oxidation products from lipid matrices has thus far been accomplished by three different methods: (i) simple liquid extraction, mainly with methanol; ,, (ii) solid phase extraction on a silica SEP-PAK cartridge washed with hexane; (iii) saponification; or (iv) conventional distillation of fatty acid methyl esters of methylated triacylglycerols to yield a residue rich in T and T oxidation products . One of the main difficulties of the enrichment of these analytes is that the oxidation products are thermolabile and undergo further degradation.…”